Stress Concentration Problems In Hollow Drill Steel

CONSIDERING that a typical modern drilling machine strikes approximately 2000 blows per min and may develop a force of 30 to 200 ft-lb, de- pending upon size of the drill used, the importance of uniform stress distribution in this type of drilling becomes obvious. If the rod were perfectly symmetrical, and if it did not rotate, the problem would be relatively simple, however, the necessary forging, machining and heat treating operations prevent attaining this state of perfection. Take the modern hollow drill rod. The blow is transmitted not from the striking end of the rod directly through to the point of impact of the bit in the rock, but rather through a somewhat complex tool. To add further problems, this entire assembly is rotated immediately after the impacting blow. Rather than attempt to discuss the magnitude and distribution of stresses, it is desired to present a few things that will minimize unusual stress concentration and produce a rod in which the stress distribution will approach the theoretical ideal.